This invention relates to a key board switch suitable for use for a computer, a type writer, a word processor and the like, and more particularly to a key board switch in which key switches constituting the key board switch each are provided with a thin film diode, resulting in exhibiting an N-key rollover function.
Conventionally, a key board switch of a key matrix structure, as shown in FIG. 5, is so constructed that a matrix is constituted by a plurality of drive lines D0, D1,--and a plurality of sense lines S0, S1,--and key switches A, B, C, D,--each are arranged in proximity to each intersection between the both lines. A voltage is applied through resistors 31 to each sense line. The ON and Off states of each of the key switches are judged by detecting a variation in voltage of each of the sense lines S0, S1,--caused when the drive lines D0, D1,--are scanned.
In the key board switch constructed as described above, when the key switch B is checked using the drive line D1 and sense line S0 while the key switches A, C and D are concurrently pushed, the key switch B is judged to be open although it is pushed because a current deflectingly flows through the key switches A, C and D as indicated at an arrow in FIG. 5.
In view of the foregoing, in the conventional key board switch, the key switches A, B, C and D each are connected thereto a diode in series to prevent such deflecting flow of a current as described above. In the art, this is called an N-key rollover function.
Diodes used for providing the key board switch with the N-key rollover function are independent from the key switches. Thus, the conventional key board switch has the following disadvantages.
First, when each of the key switches constituting the key board switch comprises a mechanical switch, it is required to mount the diode on a printed circuit board or PCB constituting a part of the mechanical switch, resulting in increasing the thickness of the PCB to a degree sufficient to cause the PCB to be unsuitable for a thin-type key board switch.
Also, the key board switch generally includes about one hundred key switches, resulting in requiring diodes corresponding in number to the key switches. This renders the assembling of the key board switch highly troublesome.
Further, when each of the key switches constituting the key board switch comprises a membrane switch, it is impossible to arrange a diode of a large thickness on the membrane switch in a manner to correspond to each of contacts of a flexible printed circuit board because the membrane switch has a small thickness. In view of such a difficulty, it is practiced that each of the diodes is provided on a separate substrate and connected through a wiring to each of the contacts on the flexible printed circuit board. Unfortunately, such construction causes the arrangement of the wiring on the flexible printed circuit board for the connection to the separate substrate to be highly complicated and the thickness of the whole key board switch in which the separate substrate and membrane switch are integrated with each other to be substantially increased.